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Date: Tue, 15 Jul 2014 09:06:00 -0500 (CDT)
From: Christoph Lameter <cl@gentwo.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
cc: Rusty Russell <rusty@rustcorp.com.au>, Tejun Heo <tj@kernel.org>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC] percpu: add data dependency barrier in percpu
 accessors and operations
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On Tue, 15 Jul 2014, Paul E. McKenney wrote:

> If I understand your initialization procedure correctly, you need at least
> an smp_wmb() on the update side and at least an smp_read_barrier_depends()
> on the read side.

A barrier for data that is not in the cache of the read side? That has
not been accessed yet (well there could have been a free_percpu before but
if so then the cache line was evicted by the initialization code).